Filtration devices using various kinds of granular media are used extensively in the treatment of water for removing all or a substantial part of various contaminants. At municipal and industrial applications these devices take the form of columns, vessels or fixed bed configurations, while at the Point-of-Use (POU) for consumer applications these are small tubular filter cartridges that can be inserted in line, in a municipal water distribution system, or more limited smaller scale water distribution system. While string wound, melt blown, or other kinds of non-woven fiber filters in similar tubular configuration are primarily intended to remove sediments from the water, filters containing granular media are designed to remove specific contaminants or impurities from the water. Depending on the nature of the contaminant, appropriate material having chemical and/or physical affinity for that particular contaminant or impurity is used. Typically the media having affinity for a particular contaminant is enclosed in a filter cartridge with an inlet port and outlet port, and the water requiring treatment is passed through the filter cartridge under some type of pressure. The contaminants should have sufficient time to contact the media, in order for the media to adsorb the contaminants thus purifying the water of that particular impurity. One of the most commonly used granular media is an activated carbon for removing color, odor and many types of dissolved organic impurities. Some other commonly used media are green sands, alumina, silica, titania, iron oxides and synthetic ion exchange resins.
Besides the POU filter devices that are attached to a pressurized water source, new types of POU filter devices known as Carafe or Pitcher filters have come into vogue, requiring the flow of water through them by gravity. These filter devices are typically used in a kitchen for removal of dissolved chlorine and to improve the taste of water either for drinking or for cooking purposes. Water, to be purified in small batches on the order of 1 to 2 liters, is poured into a reservoir containing the filter and the gravity output from the filter is collected in an another reservoir located underneath the filter. The considerations required here to select the proper particle size of the media to achieve certain flow rate and capability to reduce the contaminants from the water are quite different than for the POU filter devices that are pressurized. Consumers typically demand a flow rate of about 150 to 350 cc/min from these pitcher devices to be practical for everyday use, otherwise it takes too long to treat the 1 to 2 liters of water that the consumer needs either for drinking or cooking in an kitchen type environment.
The Problem Presented:
In a typical application involving granular media, water containing impurities or contaminants is brought into contact with the media to adsorb the contaminants onto the media and thus purify the water. The effectiveness of this operation depends on the adsorption capacity of the media for the particular impurity and the contact time during which the media has the opportunity to remove the impurity from the water. Even with the optimum adsorption capacity of the media for the impurity, if the contact time with the media is insufficient, the impurity will not be fully removed from the water. In filtration technology, Empty-Bed-Contact-Time (EBCT) expresses these kinds of solid/liquid contact considerations quantitatively. EBCT is a ratio of volume of vessel or filter cartridge containing the media to the flow rate of water through it (e.g., volume in cc of filter cartridge filled with media divided by cc/min flow rate). The resultant ratio gives time in minutes that the water is in contact with the media. It makes a big difference in the EBCT whether the treatment device is pressurized or not pressurized, as a pressurized device usually has a higher flow rate resulting in EBCT that is much less. The EBCT for pressurized systems is usually in the range of 1 to 10 seconds, whereas gravity flow systems have EBCT that is one or two orders of magnitude higher, i.e., 10 to 100 seconds. For comparison, the EBCT for large industrial and municipal applications using media in large columns and vessels is 5 to 15 minutes. Here the vessels or columns containing the media can be made sufficiently large (and they are usually arranged in series) to achieve a flow rate of 20 to 50 gallons per minute with adequate EBCT to effectively remove the contaminant. The flow rates of pressurized POU filter devices are usually in the range of 0.5 to 2 gpm, while those in pitcher filters are in the range of about 150 to 300 cc/min (0.04 to 0.08 gpm).
Even though flow rates of 150 to 300 cc/min typically found in pitcher filters allow higher contact time, they have severe size limitations. This means that only a limited quantity of granular media can be accommodated within these gravity fed filter cartridges and any slight advantage of higher contact over the pressure driven devices is nullified. This leads to an inability to remove a wide range of contaminants from these gravity driven filter devices.
One possible solution to the problem of insufficient contact time in a restrictive sized filter device is to increase the surface area of the media by using finer particle sizes. This usually achieves two purposes:
1) It increases the adsorption capacity per unit weight of the media, or more specifically per unit volume of the filter housing, and
2) It achieves better contact with the more exposed surfaces of the media. This however leads to another problem, that of reduction of flow rate under gravity. By using finer granular media, the minimum required flow rate of 150-300 cc/min is unattainable, even though the ability of the filter to remove a wide range of contaminants is vastly improved. The present invention provides a solution to this problem by the use of a novel flow facilitator that allows achievement of higher flow rate either by using finer granular media or by increasing the weight of the media, i.e., the height of the column, within the gravity driven filter cartridge.